1. Field of the Invention
This invention relates generally to polyimides. It relates particularly to self-metallizing, flexible polyimide films having highly reflective surfaces, which are prepared by an in situ self-metallization procedure.
2. Description of the Related Art
State-of-the-art reflective polymer films are currently produced by multiple step methods, such as sputter-coating, metal vapor deposition, electrodeposition or electroless chemical reduction, which involve physically adding a metal layer to the surface of a polymer film. In addition to having more than one step, this type of procedure yields reflective films in which the integrity of the metal-polymer interface is less than optimal, and because of the equipment used, the area of film that can be metallized at a given time is limited.
Metallic silver has a singularly high reflection coefficient (0.93) and thus, it is highly desirable as a reflecting material. In past work, polyimides modified with silver additives have occasionally produced metallic surfaces [A. L. Endrey, U.S. Pat. No. 3,073,784 (1963); A. Auerbach, J. Electrochem. Soc.: Accelerated Brief Comm., 937 (April 1984)]. Although the surfaces of the films prepared in these investigations were sometimes described as reflective, no quantitative measurements of this property were made. The technique of metal interlayer deposition has been used to produce reflective and conductive silver interlayers in polyimides in which the reflectivity was measured [S. Mazur, U.S. Pat. No. 4,512,855 (1985)]. Reflectivities of the metallized interlayers made in this fashion were between 6 and 36%.
Polyimides which have been reported as having metallized and/or reflective surfaces have as a rule been doped with silver concentrations as high as 25%. Silver has been shown to inhibit the imidization process, thus leading to reduced thermal stability and increased polymer degradation [J. T. Young, W. H. Tsai and F. J. Boerio, Macromolecules, 25,887 (1992)]. For this reason, although the highly-doped polyimide films of past work were suitable for decorative purposes, they were neither flexible nor thermally stable enough to be considered for use in large space structures such as metallic reflectors, concentrators, or antennas which require the ability to be packaged into relatively small volumes of low weight and subsequently unfurled upon deployment. Furthermore, the silver additive utilized in most of the past studies was silver nitrate. Other silver additives are not soluble in polyamic acid resins, and either cause gellation or precipitate out of solution. Silver nitrate, although readily soluble in polyamic acid resins, produces brittle films.